Polyphosphazene blends

ABSTRACT

Polyphosphazene blends and foams thereof are described. The blends comprise of at least one polyphosphazene copolymer having a Young&#39;s Modulus of up to about 5×10 8  dynes/cm 2  and at least one polyphosphazene homopolymer or copolymer having a Young&#39;s Modulus of at least about 5×10 8  dynes/cm 2 . The copolymers used to prepare the blends comprise randomly repeating units represented by the formulas ##STR1## wherein R 1  and R 2  are the same or different and are hydrogen, a C 1  - C 10  linear or branched alkyl radical, or a C 1  - C 4  linear or branched alkoxy radical substituted on any sterically permissible position on the phenoxy group, with the proviso that when R 2  is alkoxy, R 1  and R 2  are different. The blends of this invention can be formed into sheets or films, or into flexible or semi-rigid foams. The blends are extremely fire retardant and produce low smoke loads, or essentially no smoke, when heated in an open flame.

DESCRIPTION OF THE INVENTION

This invention relates to blends of polyphosphazene homopolymers andcopolymers, to flexible and semirigid foams produced from said blends,and to a process for preparing said blends and foams. The blends of thisinvention exhibit excellent flame retardant and film-forming properties.Foams prepared from the blends exhibit excellent flame retardantproperties and produce low smoke levels, or essentially no smoke, whenheated in an open flame.

The preparation of polyphosphazene polymers has been disclosed in Pat.No. 3,370,020 to Allcock et al., 3,856,712 to Reynard et al., 3,856,713to Rose et al., and 3,883,451 to Reynard et al. Similarly, the conceptof blending phosphazene-epoxy prepolymers with organic liquidprepolymers such as phenolics, epoxies, polyurethane and polyesters, andsubsequently curing such blends is disclosed in 3,867,344 to Frank etal. However, the products produced by the methods of the prior art havewidely varying physical characteristics which, in many cases, limittheir utility, particularly when it is desired to prepare flexible orsemirigid foams.

We have now found that products, particularly films and foams, havingtailored, highly desirable physical characteristics may be prepared byblending two or more polyphosphazene homopolymers or copolymers havingwidely differing degrees of elasticity. That is to say, we havediscovered that films and foams having a predetermined degree offlexibility, and exhibiting excellent flame retardant and smokeproperties, may be prepared by blending at least one relativelyelastomeric polyphosphazene copolymer having a Young's Storage Modulusof up to about 5×10⁸ dynes/cm with at least one relatively stiff orrigid polyphosphazene homopolymer or copolymer having a Young's Modulusof at least about 5×10⁸ dynes/cm. The ratio of the relativelyelastomeric polymer to the relatively stiff polymer may vary over a widerange, with ratios in the range of from about 1:3 to about 3:1 beingpreferred.

The polyphosphazene polymers used to prepare the blends of thisinvention comprise randomly repeating units represented by the formulas##STR2## wherein R₁ and R₂ are the same or different and are hydrogen, aC₁ - C₁₀ linear or branched alkyl radical, or a C₁ - C₄ linear orbranched alkoxy radical substituted on any sterically permissibleposition on the phenoxy group, with the proviso that when R₂ is alkoxyand when copolymers are to be prepared, R₁ and R₂ are different.Examples of R₁ and R₂ include ethoxy, methoxy, isopropoxy, n-butoxy,methyl, ethyl, n-propyl, isopropyl, sec-butyl, tert-butyl, tert-pentyl,2-ethylhexyl and n-nonyl.

It is to be understood that when R₁ is the same as R₂, homopolymers areformed. Further, it is to be understood that while it is presentlypreferred that all R₁ 's are the same all R₂ 's are the same, the R₁ 'scan be mixed and the R₂ 's can be mixed. The mixtures may be mixtures ofdifferent alkyl radicals or mixtures of different ortho-, meta- andpara- isomers. One skilled in the art readily will recognize that sterichindrance will dictate the propriety of using relatively bulky groups inthe para-position on the phenoxy ring since as set forth hereinafter thepolymers are made by reacting a substituted metal phenoxide with achlorine atom on a phosphrous atom. Desirably, groups which stericalkyinhibit this reaction should be avoided. Absent the foregoing proviso,the selection of the various R₁ 's and R₂ 's will be apparent to anyoneskilled in the art based upon this disclosure.

For the sake of simplicity, the polymers used to prepare the blends ofthe invention which contain the above three repeating units may berepresented by the formula [NP (OC₆ H₄ -R₁)_(a) (OC₆ H₄ -R₂)_(b) ]_(n),wherein n is from about 20 to about 2,000 or more, and wherein a and bare greater than zero and a+b=2.

The above described polymers, as well as those containing reactive sitesdesignated as W below, may be crosslinked and/or cured at moderatetemperatures (for example, 200°-350° F.) by the use of free radicalinitiators, for example, peroxides, using conventional amounts,techniques and processing equipment.

The copolymers used to prepare the blends of this invention may containsmall amounts of randomly distributed repeating units in addition to therepeating unites described above. Examples of these additional repeatingunits are: ##STR3## wherein W represents a group capable of acrosslinking chemical reaction, such as, an olefinically unsaturated,preferably ethylinically unsaturated monovalent radical, containing agroup capable of further reaction at relatively moderate temperatures,and the ratio of W:[(-OC₆ H₄ -R₁)+ (-OC₆ H₄ -R₂)] is less than about1:5. For the sake of simplicity, the copolymers which are furtherreactive may be represented by the formula [NP(OC₆ H₄ -R₁)_(a) (OC₆ H₄-R₂)_(b) (W)_(c) ]_(n), wherein W, R₁, R₂, n, a and b are as set forthabove, and wherein a+b+c=2. Examples of W are -OCH=CH_(;) -OR₃ CH=CH₂ ;##STR4## OR₃ CF=CF₂ and similar groups which contain unsaturation, whereR₃ is any aliphatic or aromatic radical, especially -CH₂ -. These groupsare capable of further reaction at moderate temperatures (for example,200°-350° F.) in the presence of free radical initiators, conventionalsulfur curing or vulcanizing additives known in the rubber art or otherreagents, often even in the absence of accelerators, using conventionalamounts, techniquesand processing equipment.

Examples of free radical initiators include benzoyl, peroxide,bis(2,4-dichlorobenzoyl peroxide), di-tert-butyl peroxide, dicumylperoxide, 2,5-dimethyl (2,5-di-tert-butylperoxy) hexane, t-butylperbenzoate, 2,5-dimethyl-2,5-di(tert-butyl peroxy) hepyne-3, and1,1-bis (tert-butylperoxy)-3,3,5-trimethylcyclohexane. Thus, the generalperoxide classes which may be used for crosslinking include diacylperoxides, peroxyesters, and dialkyl peroxides.

Examples of sulfur-type curing systems include vulcanizing agents suchas sulfur, sulfur monochloride, selenium, tellurium, thuriramdisulfides, p-quinone dioximes, polysulfide polymers, and alkyl phenolsulfides. The above vulcanizing agents may be used in conjunction withaccelerators, such as aldehyde amines, thio carbamates, thiuramsulfides, quanidines, and thiazols, and accelerator activators, such aszinc oxide or fatty acids, e.g., stearic acid.

It is also possible to use as W in the above formulas, monovalentradicals represented by the formulas (1) -OSi(OR⁴)₂ R⁵ and other similarradicals which contain one or more reactive groups attached to silicon;(2) -OR⁶ NR₆ H and other radicals which contain reactive -NH linkages.In these radicals R⁴, R⁵ and R⁶ each represent aliphatic, aromatic andacyl radicals. Like the groups above, these groups are capable offurther reaction at moderate temperatures in the presence of compoundswhich effect crosslinking. The presence of a catalyst to achieve a cureis often desirable. The introduction of groups such as W intopolyphosphazene polymers is shown in U.S. Pat. Nos. 3,888,799; 3,702,833and 3,844,983, which are hereby incorporated by reference. It iscontemplated that the copolymers used to prepare the blends of thisinvention contain a mole ratio of a:b at least about 1:6 and up to 6:1,and preferably betwen about 1:4 and 4:1. It is also contempated that themole ratio of c:(a+b) will be less than about 1:5, and preferably fromabout 1:50 to about 1:10.

In one embodiment, the polymers used to prepare the blends of thisinvention may be prepared in accordance with the process described inour copending application Ser. No. 661,862, filed Feb. 27, 1976, whichdescription is incorporated herein by reference. Accordingly, thepolymers which may be used to prepare the blends of this invention maybe prepared by multistep process wherein the first step comprisesthermally polymerizing a compound having the formula

    (NPCl.sub.2).sub.3

by heating it at a temperature and for a length of time ranging fromabout 200° C. for 48 to 300° C. for 30 minutes, preferably in theabsence of oxygen, and most preferably in the presence of a vacuum of atleast 10⁻¹ Torr. That is to say, the compounds are heated to atemperature ranging from about 200° C to about 300° C. for from about 30minutes to 48 hours, the higher temperatures necessitating shortercontact times and the lower temperatures necessitating longer contacttimes. The compounds must be heated for such a length of time that onlya minor amount of unreacted charge material remains and a major amountof high polymer has been produced. Such a result is generally achievedby following the conditions of temperature and contact time specifiedabove.

It is preferred that the thermal polymerization be carried out in thepresence of an inert gas such as nitrogen, neon, argon or a vacuum,e.g., less than about 10⁻¹ Torr insasmuch as the reaction proceeds veryslowly in the presence of air. The use of such gas, however, is notcritical.

The polymers resulting from the thermal polymerization portion of theprocess are in the form of a polymeric mixture of different polymers ofdifferent chain lengths. That is to say, the product of the thermalpolymerization is a mixture of polymers having the formula

    --NPCl.sub.2).sub.n

wherein n ranges from about 20 to about 2000. For example, the recoveredmedia may contain minor amounts of a polymer where n is 20 and majoramounts of polymer where n is 2000. The media may also contain polymerscomposed of from 21-1999 recurring units and some unreacted trimer. Thecomplete mixture of polymers and unreacted trimer constitutes the chargeto the second step of the process.

When homopolymers are to be prepared, the second or esterification stepof the process comprises treating the mixture resulting from the thermalpolymerization step with a compound having the formula

    M(OC.sub.6 H.sub.4 -R.sub.2).sub.x

wherein M is lithium, sodium, potassium, magnesium or calcium, x isequal to the valence of metal M, and R₂ is as specified above.

Similarly, when copolymers are to be prepared, the second oresterification step comprises treating the mixture resulting from thethermal polymerization step with a mixture of compounds having theformulas

    M(OC.sub.6 H.sub.4 -R.sub.1).sub.x

    M(OC.sub.6 H.sub.4 -R.sub.2).sub.x and, if desired,

    M(W).sub.x

wherein M, x, R₁, R₂ and W are as specified above, with the proviso thatwhen R₂ is alkoxy, R₂ and R₁₂₁ are different.

Regardless of whether homopolymers or copolymers are being prepared, thepolymer mixture is reacted with the above described metal compound ormixture of metal compounds at a temperature and a length of time rangingfrom about 25° C. for 7 days to about 200° C. for 3 hours

Again, as in regard to the polymerization step mentioned above, thepolymer mixture is reacted with the alkali or alkaline earth metalcompounds at a temperature ranging from about 25° C. to about 200° C.for from about 3 hours to 7 days, the lower temperatures necessitatingthe longer reaction times and the higher temperatures allowing shorterreaction times. These conditions are, of course, utilized in order toobtain the most complete reaction possible, i.e., in order to insure thecomplete conversion of the chlorine atoms in the polymer mixture to thecorresponding ester of the alkali or alkaline earth starting materials.

The above esterification step is carried out in the presence of asolvent. The solvent employed in the esterification step must have arelatively high boiling point (e.g., about 115° C., or higher) andshould be a solvent for both the polymer and the alkali or alkalineearth meta compounds. In addition, the solvent must be substantiallyanhydrous, i.e., there must be no more water in the solvent or metalcomponds than will result in more than 1%, by weight, of water in thereaction mixture. The prevention of water in the system is necessary inorder to inhibit the reaction of the available chlorine atoms in thepolymer therewith. Examples of suitable solvents include diglyme,triglyme, tetraglyme, toluene and xylene. The amount of solvent employedis not critical and any amount sufficient to solubilize the chloridepolymer mixture can be employed. Either the polymer mixture or thealkaline earth (or alkali) metal compounds may be used as a solventsolution thereof in an inert, organic solvent. It is preferred, however,that at least one of the charge materials be used as a solution in acompound which is a solvent for the polymeric mixture.

The amount of the alkali metal or alkaline earth metal compound employedor the combined amount of the mixture of said compounds employed whencopolymers are being prepared should be at least molecularly equivalentto the number of available chlorine atoms in the polymer mixture.However, it is preferred that an excess of the metal compounds beemployed in order to assure complete reaction of all the availablechlorine atoms. Generally, the ratio of the individual alkali metal oralkaline earth metal compounds in the combined mixture governs the ratioof the groups attached to the polymer backbone. However, those skilledin the art readily will appreciate that the nature and, moreparticularly, the steric configuration of the metal compounds employedmay effect their relative reactivity. Accordingly, when preparingcopolymers, the ratio of R₁ 's and R₂ 's in the esterified product, ifnecessary, may be controlled by employing a stoichiometric excess of theslower reacting metal compound.

Examples of alkali or alkaline earth metal compounds which are useful inthe process of the present invention includ

sodium phenoxide

potassium phenoxide

sodium p-methoxyphenoxide

sodium o-methoxyphenoxide

sodium m-methoxyphenoxide

lithium p-methoxyphenoxide

lithium o-methoxyphenoxide

lithium m-methoxyphenoxide

potassium p-methoxyphenolic

potassium o-methoxyphenoxide

potassium m-methoxyphenoxide

magnesium p-methoxyphenoxide

magnesium o-methoxyphenoxide

magnesium m-methoxyphenoxide

calcium p-methoxyphenoxide

calcium o-methoxyphenoxide

calcium m-methoxyphenoxide

sodium p-ethoxyphenoxide

sodium o-ethoxyphenoxide

sodium m-ethoxyphenoxide

potassium p-ethoxyphenoxide

potassium o-ethoxyphenoxide

potassium m-ethoxyphenoxide

sodium p-n-butoxyphenoxide

sodium m-n-butoxyphenoxide

lithium p-n-butoxyphenoxide

lithium m-n-butoxyphenoxide

potassium p-n-butoxyphenoxide

potassium m-n-butoxyphenoxide

magnesium p-n-butoxyphenoxide

magnesium m-n-butoxyphenoxide

calcium p-n-butoxyphenoxide

calcium m-n-butoxyphenoxide

sodium p-n-propoxyphenoxide

sodium o-n-propoxyphenoxide

sodium m-n-propoxyphenoxide

potassium p-n-propoxyphenoxide

potassium o-n-propoxyphenoxide

potassium m-n-propoxyphenoxide

sodium p-methylphenoxide

sodium o-methylphenoxide

sodium m-methylphenoxide

lithium p-methylphenoxide

lithium o-methylphenoxide

lithium m-methylphenoxide

sodium p-ethylphenoxide

sodium o-ethylphenoxide

sodium m-ethylphenoxide

potassium p-n-propylphenoxide

potassium o-n-propylphenoxide

potassium m-n-propylphenoxide

magnesium p-n-propylphenoxide

sodium p-isopropylphenoxide

sodium o-isopropylphenoxoide

sodium m-isopropylphenoxide

calcium p-isopropylphenoxide

calcium o-isopropylphenoxide

calcium m-isopropylphenoxide

sodium p-sec butylphenoxide

sodium m-sec butylphenoxide

lithium p-sec butylphenoxide

lithium m-sec butylphenoxide

lithium p-tert. butylphenoxide

lithium m-tert. butylphenoxide

potassium p-tert. butylphenoxide

potassium m-tert. butylphenoxide

sodium p-tert. butylphenoxide

sodium m-tert. butylphenoxide

sodium propenoxide

sodium p-nonylphenoxide

sodium m-nonylphenoxide

sodim o-nonylphenoxide

sodium 2-methyl-2-propeneoxide

potassiun buteneoxide

and the like.

The second step of the process results in the production of ahomopolymer mixture having the formula

    [NP(OC.sub.6 H.sub.4 -R.sub.2 ].sub.n

or a copolymer mixture having the formula

    [NP(OC.sub.6 H.sub.4 R.sub.1).sub.a (OC.sub.6 H.sub.4 R.sub.2).sub.b (W).sub.c ].sub.n

wherein n, R₁, R₂ and W are as specified above, where c, but not a and bcan be zero, and where a+b+c=2, and the corresponding metal chloridesalt.

The polymeric reaction mixture resulting from the second oresterification step is then treated to remove the salt which resultsupon reaction of the chlorine atoms of the polymer mixture with themetal of the alkali or alkaline earth metal compounds. The salt can beremoved by merely precipitating it out and filtering, or it may beremoved by any other applicable method, such as by washing the reactionmixture with water after neutralization thereof with, for example, anacid such as hydrochloride acid.

The next step in the process comprises fractionally precipitating thepolymeric material to separate out the high polymer from the low polymerand any unreacted trimer. The fractional precipitation is achieved bythe, preferably dropwise, addition of the esterified polymer mixture toa material which is a non-solvent for the high polymer and a solvent forthe low polymer and unreacted trimer. This is to say, any material whichis a non-solvent for the polymers wherein n is higher than 350 and asolvent for the remaining low polymers may be used to fractionallyprecipitate the desired polymers. Examples of materials which can beused for this purpose include hexane, diethyl ether, carbontetrachloride, chloroform, dioxane methanol, water and the like. Thefractional precipitation of the esterified polymeric mixture generallyshould be carried out at least twice and preferably at least four timesin order to remove as much of the low polymer from the polymer mixtureas possible. The precipitation may be conducted at any temperature,however, it is preferred that room temperature be employed. The highmolecular weight polymer mixture may then be recovered by filtration,centrifugation, decantation or the like.

The homopolymers and copolymers prepared in accordance with the abovedescribed process are thermally stable. They are soluble in specificorganic solvents such as tetrahydrofuran, benzene, xylene, toluene,dimethylformamide and the like and can be formed into films fromsolutions of the polymers by evaporation of the solvent. The polymers arwater resistant at room temperature and do not undergo hydrolysis.However, the elasticity of the various polymers varies greatly, suchthat many of the polymers can not be worked into useful forms. Thisundesirable characteristic can be overcome by blending at least one ofthe above polymers having a Young's Storage Modulus in the range of1×10⁶ to 5×10⁸ dynes/cm², preferably 4×10⁶ to 7×10⁷ dynes/cm² with atleast one of the above polymers having a Young's Modulus in the range of5×10⁸ to 6×10¹⁰ dynes/cm², preferably 2.44×10⁹ to 2.61×10¹⁰ dynes/cm²,at a blend ratio of from about 1:3 to 3:1. The resulting blends arecharacterized by a Young's Modulus between the values of theirrelatively elastomeric and stiff components. The blends may be used toprepare films, fibers, coatings, molding compositions and the like.Additionally, the blends may be used to prepared foamed products whichexhibit excellent fire retardance and which produce low smoke levels, oressentially no smoke when heated in an open flame. The foamed productsmay be prepared from filled or unfilled formulations using conventionalfoam techniques with chemical blowing agents, i.e., chemical compoundsstable at original room temperature which decompose or interact atelevated temperatures to provide a cellular foam. Suitable chemicalblowing agents include:

    ______________________________________                                                                  Effective                                                                     Temperature                                         Blowing Agent             Range ° C.                                   ______________________________________                                        Azobisisobutyronitrile    105-120                                             Azo dicarbonamide (1,1-azobisform-                                             amide)                   100-200                                             Benzenesulfonyl hydrazide  95-100                                             N,N'-dinitroso-N,N'-dimethyl tere-                                             phthalamide               65-130                                             Dinitrosopentamethylenetetramine                                                                        130-150                                             Ammonium carbonate        58                                                  p,p'-oxybis-(benzenesulfonyl-                                                  hydrazide)               100-200                                             Diazo aminobenzene        84                                                  Urea-biuret mixture        90-140                                             2,2'-axo-isobutyronitrile  90-140                                             Azo hexahydrobenzonitrile  90-140                                             Diisobutylene             103                                                 4,4'-diphenyl disulfonylazide                                                                           110-130.                                            ______________________________________                                    

Typical foamable formulations include:

    ______________________________________                                        Polyphosphazene elastomer                                                                              50       parts                                       Polyphosphazene nonelastomer                                                                           50       parts                                       Filler (e.g., alumina trihydrate)                                                                      0-100    phr                                         Stabilizer (e.g., magnesium oxide)                                                                     2.5-10   phr                                         Processing aid (e.g., zinc stearate)                                                                   2.5-10   phr                                         Plasticizer resin (e.g., Cumar P-10,                                           coumarone indene resin) 0-50     phr                                         Blowing agent (e.g., 1,1'-azobisformamide)                                                             10-50    phr                                         Activator (e.g., oil-treated urea)                                                                     10-40    phr                                         Peroxide curing agent (e.g., 2,5-                                              dimethyl-2,5-di(t-butylperoxy)                                                hexane)                 2.5-10   phr                                         Peroxide curing agent (e.g., benzoyl                                           peroxide)               2.5-10   phr                                         ______________________________________                                    

While the above are preferred formulation guidelines, obviously some orall of the adjuvants may be omitted, replaced by other functionallyequivalent materials, or the proportions varied, within the skill of theart of the foam formulator.

In one suitable process, the foamable ingredients are blended togetherto form a homogeneous mass; for example, a homogeneous film or sheet canbe formed on a 2-roller mill, preferably with one roll at ambienttemperature and the other at moderately elevated temperature, forexample 20°-40° F. The homogeneous foamable mass can then be heated, toprovide a foamed structure; for example, by using a mixture of a curingagent having a relatively low initiating temperature, such as benzoylperoxide, and a curing agent having a relatively high initiatingtemperature, such as 2,5-dimethyl-2,5-di(t-butylperoxy) hexane, andpartially pre-curing in a closed mold for about 6-30 minutes at200°-250° F.; following by free expansion for 30-60 minutes at 300°-350°F. In the alternative, the foaming may be accomplished by heating thefoamable mass for 30-60 minutes at 300°-350° F. using a high temperatureor low temperature curing agent, either singly or in combination. Onebenefit of utilizing the "partial pre-cure" foaming technique is that anincrease in the molecular weight of the foamable polymer prior to thefoaming step enables better control of pore size and pore uniformity inthe foaming step. The extent if "pre-cure" desired is dependent upon theultimate foam characteristics desired. The desired foaming temperatureis dependent on the nature of the blowing agent and the crosslinkerspresent. The time of heating is dependent on the size and shape of themass being foamed. The resultant foams are generally light tan toyellowish in appearance, and vary from flexible to semirigid, dependingupon the relative amounts and the Young's Modulus of the elastomeric andnonelastomeric polymers employed in the foam formulation. As indicated,inert, reinforcing or other fillers such as alumina trihydrate, hydratedsilicas or calcium carbonate can be added to the foams and the presenceof these and other conventional additives should in no way be construedas falling outside the scope of this invention.

Also, as mentioned above, the blends of this invention can becrosslinked at moderate temperatures by conventional free radical and/orsulfur curing techniques when minor amounts of unsaturated groups W arepresent in the copolymer backbone. The ability of these blends to becured at temperatures below about 350° F. makes them particularly usefulas potting and encapsulation compounds, sealants, coatings and the like.These blends are also useful for preparing crosslinked foams whichexhibit significantly increased tensile strengths over uncured foams.These blends are often crosslinked in the presence of inert, reinforcingor other fillers and the presence of these and other conventionaladditives are deemed to be within the scope of this invention.

The following examples are set forth for purposes of illustration onlyand are not to be construed as limitations of the present inventionexcept as set forth in the appended claims. All parts and percentagesare by weight unless otherwise indicated.

EXAMPLE 1 Preparation of --NPCl₂)_(n)

250 parts of phosphonitrilic chloride trimer, previously recrystallizedfrom n-heptane, were degassed and sealed in a suitable, thick-walledreaction vessel at 10⁻² Torr and heated to 250° C. for 6 hours.Polymerization was terminated at this time since a glass ball, one-halfinch in diameter ceased to flow due to the increased viscosity of themolten mass, when the vessel was inverted. Termination as effected bycooling the vessel to room temperature. The resulting polymeric mixturewas then dissolved in toluene to form an anhydrous solution.

EXAMPLE 2 Preparation of [NP(OC₆ H₄ -4-sec C₄ H₉) (OC₆ H₅)]_(n)

The anhydrous toluene solution of poly (dichlorophosphazene) formed inExample 1, containing 0.97 equivalents of poly(dichlorophosphazene), wasadded to an anhydrous diglymebenzene solution of 0.62 equivalents ofNaOC₆ H₄ -sec C₄ -H₉ and 0.62 equivalents of NaOC₆ H₅ at a temperatureof 95° C. with constant stirring. After the addition, benzene wasdistilled from the reaction mixture until a temperature of 115°-116° C.was attained. The reaction was then heated at reflux for 60-65 hours. Atthe end of this time the copolymer was precipitated by pouring thereaction mixture into an excess of methyl alcohol. The polymer wasstirred in the methyl alcohol for 24 hours. Next, it was added to alarge excess of water and stirred for an additional 24 hours. Theresulting product (up to 62 percent yield) was an elastomeric solidhaving a glass transition temperature (Tg) of -8.1° C. and a Young'sStorage Modulus of 1×10⁷ dyne/cm². The Young's Modulus was determinedusing a Rhevibron tensile tester (Toyo Measuring Instrument Co., E. A.Tolle Co., Hingham, Mass.) which measures the dynamic tensile modulus byoscillating a sample in tension. The product was soluble in benzene,tetrahydrofuran and dimethylformamide. The copolymer mixture was thencast to a tough, transparent film from solution in tetrahydrofuran. Thefilm was flexible, did not burn, and was water-repellent. The copolymerhad an Oxygen Index (OI) of 25.9 as determined according to theprocedure described in ASTM D-2863-74, "Flammability of Plastics Usingthe Oxygen Index Method". By this method, material samples, which are 6× 2 × 0.01 to 0.03 inch , are held in a U-shaped frame and the burningof the samples under a specific set of conditions is measured. It hasbeen shown that this technique actually measures the lowest oxygenconcentration in an atmosphere which will just prevent sustained burningof a top-ignited sample (see Fenimore et al, Combustion and Flame, 10,135 (1966)). The oxygen index values also have been related to thetemperature at which a mixture of fuel and a controlled flow of oxygenwill just burn when the fuel is composed of volatile pyrolytic productsor fragments (see, Johnson et al, Rubber Age, 107 (No. 5), 29 (1975)).Analysis: Calculated (percent) for 1:1 copolymer of [NP(OC₆ H₄ -4-sec C₄H₉) (OC₆ H₅)]_(n) : C, H, N, and Cl, 0.00. Found (percent): C, H. N, andCl, 0.00

EXAMPLE 3

Polyphosphazene homopolymers and copolymers were preparedn by amultistep process beginning with the thermal polymerization ofhexachlorocyclotriphosphazene, N₃ P₃ Cl₆, as described in Example 1. Theresulting poly(dichlorophosphazene) [NPCl₂ ]_(n) was dissolved in asuitable solvent, such as toluene. This polymeric solution was thenadded to a bis(2-methoxyethyl) ether solution of the desired sodiumaryloxide salt at 95° C. (Copolymers were prepared by adding the polymerto a solution containing a 1:1 mole ratio of the two desired sodiumaryloxide salts.) The reaction temperature was raised to 115°-116° C.and maintained for 50-65 hours with constant stirring. The thermalpolymerization and subsequent reaction are summarized in Equations (1)and (2): ##STR5## After the reaction was completed, the polymers wereprecipitated by pouring the reaction mixture into an excess of methanol,were washed for 24 hours in methanol, and finally were exhaustivelywashed with distilled water. The polymers ranged from rigid fiber-likematerials to elastomers and, except for a few cases, were colorless. Thepolymers prepared, their glass-transition temperatures and their Young'sModulus are listed in Table I. Analytical data were in agreement withthe tabulated empirical formulas.

                                      TABLE 1                                     __________________________________________________________________________    GLASS TRANSITION TEMPERATURES * AND                                           YOUNG'S MODULUS ** OF POLYPHOSPHAZENE POLYMERS                                         [NP(OR') (OR")].sub.n                                                                      Tg. ° C.                                                                     Young's Modulus dynes/cm.sup.2                    __________________________________________________________________________    R' = C.sub.6 H.sub.5                                                                   R' =                                                                              C.sub.6 H.sub.4 -4-secC.sub.4 H.sub.9                                                  -8.1    1×10.sup.7                                C.sub.6 H.sub.5                                                                            C.sub.6 H.sub.4 -C.sub.6 H.sub.9                                                       --      5×10.sup.6a                               C.sub.6 H.sub.4 -4-OCH.sub.3                                                               C.sub.6 H.sub.4 -4-secC.sub.4 H.sub.9                                                  -5.03   7×10.sup.7                                C.sub.6 H.sub.4 -4-OCH.sub.3                                                               C.sub.6 H.sub.4 -4-C.sub.9 H.sub.19                                                    -2.23   4×10.sup.6a                               C.sub.6 H.sub.5                                                                            C.sub.6 H.sub.5                                                                        -7.7    7.3×10.sup.9                              C.sub.6 H.sub.4 -4-CH.sub.3                                                                C.sub.6 H.sub.4 -4-CH.sub.3                                                            +2.0    2.99×10.sup.9                             C.sub.6 H.sub.4 -4-isoC.sub.3 H.sub.7                                                      C.sub.6 H.sub.4 -4-isoC.sub.3 H.sub.7                                                  -0.10   7.38×10.sup.9                             C.sub.6 H.sub.4 -4-tertC.sub.4 H.sub.9                                                     C.sub.6 H.sub.4 -4-tertC.sub.4 H.sub.9                                                 +44     2.61×10.sup.10                            C.sub.6 H.sub.4 -4-OCH.sub.3                                                               C.sub.6 H.sub.4 -4-OCH.sub.3                                                           +0.60   5.03×10.sup.9                             C.sub.6 H.sub.5                                                                            C.sub.6 H.sub.4 -4-tertC.sub.4 H.sub.9                                                 +22     2.45×10.sup.9                             C.sub.6 H.sub.4 -4-OCH.sub.3                                                               C.sub.6 H.sub.4 -4-tertC.sub.4 H.sub.9                                                 +24.1   5.04×10.sup.9                             __________________________________________________________________________      * Determined by differential scanning calorimetry. The above values are      based on Idium standard (melt temperature 156.6° C.)                   ** Determined by Rheovibron instrument, 110 Hertz, 22° C.              .sup.a Estimated, sample too elastomeric for accurate measurement.       

The polymers set forth in Table 1 having a Young's Modulus from 7×10⁷dynes/cm² and below were elastomers, whereas the polymers having aYoung's Modulus of 2.45×10⁹ dynes/cm² and above were rigid ornonelastomeric fiber-like materials. The nonelastomeric materialsexhibited excellent fire retardant and smoke properties, but they couldnot sheeted or formed into foamed articles.

EXAMPLE 4

Various 1:1 blends were prepared from the elastomeric and nonelastomericpolymers set forth in Table 1. The blends were prepared using a two rollresearch mill with one roll heated to approximately 120°-140° F. and theother at ambient conditions. The specific polymers blended, the glasstransition temperature of the blends, and the Young's Modulus of theblends are set forth in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    GLASS TRANSITION TEMPERATURE *                                                AND YOUNG'S MODULUS**                                                         OF POLYPHOSPHAZENE POLYMER BLENDS                                             Polymers Blended 1:1       Young's Modulus                                    Mole Ratio            Tg° C.                                                                      dyne/cm.sup.2                                      __________________________________________________________________________    [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -sec C.sub.4 H.sub.9)].sub.n /         [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                          -5     4.8×10.sup.8                               [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].sub.n /       [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-tert C.sub.4 H.sub.9)].sub.n                              +15    1.58×10.sup.9                              [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].sub.n /       [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-tert C.sub.4           H.sub.9)].sub.n       +20    3.67×10.sup.9                              [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-sec C.sub.4            H.sub.9)].sub.n /                                                             [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                          -5     5.2×10.sup.8                               [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-sec C.sub.4            H.sub.9)].sub.n /                                                             [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-tert C.sub.4 H.sub.9)].sub.n                              +11    1.60×10.sup.9                              [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-sec C.sub.4            H.sub.9)].sub.n /                                                             [NP(CO.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-tert C.sub.4           H.sub.9)].sub.n       +15    1.75×10.sup.9                              __________________________________________________________________________      * Determined by differential scanning calorimetry. The above values are      based on Indium standard (melt temperature 156.6° C.)                  ** Determined b Rheovibron Instrument, 110 Hertz, 22° C.          

The glass transition temperature data in Table 2 indicates that thepolymer blends approach true solutions, i.e., instead of two separateglass transition temperatures, only one broad glass transition isobserved. The Young's Modulus of each blend is between the value ofeither component from which the respective blends are prepared. Eachblend is less elastomeric than its elastomeric component, but moreelastomeric than its stiff or nonelastomeric component. Each blend iscapable of being worked into a sheet or film, and each blend can befoamed.

EXAMPLE 5

Foams of filled polyphoshazene blends were prepared by blending thepolymer portion of the "standard foam recipe" on a two roll researchmill with one roll heated to 120°-140° F. and the other at ambientconditions. The polymers were blended for 15 minutes to insurehomogeneous mixing, whereafter the remaining ingredients of the"standard foam recipe" were added to the polymer blend on the researchmill. Mixing was continued for another 15 minutes to form an unexpandedblend. The unexpanded blend was then precured in a press for 12 minutesat a temperature of 220° F and a pressure of 2000 p.s.i. to form aprecured pad. Finally, the precured pad was free expanded in acirculating air oven for 30 minutes at 300° F. The "standard foamrecipe" is set forth in Table 3.

                  TABLE 3                                                         ______________________________________                                        STANDARD FOAM RECIPE                                                          Ingredient                 Amount, gm                                         ______________________________________                                        Polyphosphazene Elastomer  5g                                                 Polyphosphazene Nonelastomer                                                                             5g                                                 Alumina Trihydrate         10g                                                1,1'-Azo Bisformamide      2g                                                 BIK-OT.sup.a               0.5g                                               Magnesium Oxide            0.5g                                               Zinc Stearate              1.0g                                               Cumar.sup.b                0.2g                                               2,5-Dimethyl-2,5-Ditertiary Peroxy Hexane                                                                0.6g                                               Benzoyl Peroxide (78% Active)                                                                            0.2g                                               Dicumyl Peroxide           0.1g                                               ______________________________________                                         .sup.a UniRoyal Oil-Treated Urea (Activator)                                  .sup.b Allied Chemical p-Commarone-Indine Resin                          

EXAMPLES 6-15

Using the method and the recipe set forth in Example 5, the polymersindicated in Table 4 were formed into foamed pads. The pads were die-cutto 3×3×0.02.03 inches and were conditioned for 48 hours at 73° F. and50% relative humidity prior to testing for smoke evolution properties.The smoke evolution properties of the samples were evaluated by using anAminco-NBS Smoke Density Chamber (Model 4-5800, Aminco-NBS Smoke DensityChamber, American Instrument Co.), as described by Gross et al, "AMethod of Measuring Smoke Density from Burning Materials", ASTM STP-422(1967). Samples were tested using the flaming test mode. This smallscale test subjects a sample to the general conditions which prevail inthe majority of "real" fires and especially in tunnel tests. In thetests the maximum specific optical density Dm, corrected for sootdeposites on the cell windows was measured, and a smoke value per gram,SV/g, or Dm(corr)/g of sample was calculated. This allows for correctionof the smoke density value for its sample weight, since the samples arequite thin. Generally, NBS smoke values of 450 or less are normallyrequired in those fire or code regulations restricting smoke evolution.Values of 200 or less are uncommon for most organic polymers; those lessthan 100 are quite rare. The smoke properties of the polymer blends areset forth in Table 4, along with the smoke properties of severalcommercial polymers. The density and relative flexibility of the polymerblend foams are set forth in Table 5.

                                      TABLE 4                                     __________________________________________________________________________    NBS SMOKE DENSITY TEST RESULTS                                                POLYPHOSPHAZENE BLENDS AND REFERENCE POLYMERS                                         Reference            Flaming Mode (F)                                 Example No.                                                                           Polymer              DM(corr)                                                                            SV/g                                       __________________________________________________________________________            Polyethylene         150   --                                                 Polystyrene          468   --                                                 Poly(vinyl chloride) 530   --                                                 Polycarbonate        660   --                                                 ABS-Rubber           180   --                                                 Silicone Rubber (GE-SE9035)                                                                        385   --                                         6       [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].su            b.n /                                                                         [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         185   10                                         7       [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].su            b.n /                                                                         [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-tert C.sub.4 H.sub.9)].s            ub.n                 109   15                                         8       [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].su            b.n /                                                                         [NP(OC.sub.6 H.sub.4 -4-OC.sub.3)(OC.sub.6 H.sub.4 -tert C.sub.4              H.sub.9)].sub.n      163   36                                         9       [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-C.sub.9 H.sub.19)].sub.n             /                                                                            [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         109    8                                         10      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-sec                    C.sub.4 H.sub.9)].sub.n /                                                     [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                          95    7                                         11      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-sec                    C.sub.4 H.sub.9)].sub.n /                                                     [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-tert C.sub.4 H.sub.9)].s            ub.n                 231   14                                         12      [NP(OC.sub.6 H.sub.4 -OC.sub.3)(OC.sub.6 H.sub.4 -4-sec C.sub.4               H.sub.9)].sub.n /                                                             [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-tert                   C.sub.4 H.sub.9)].sub.n                                                                            116   14                                         13      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-C.sub.9                H.sub.19)].sub.n /                                                            [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         118    8                                         14      [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].su            b.n /                                                                         [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-OCH.sub.3)].sub.n           15      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3)(OC.sub.6 H.sub.4 -4-C.sub.9                H.sub.19)].sub.n /                                                            [NP(OC.sub.6 H.sub.5)(OC.sub.6 H.sub.4 -4-C.sub.2 H.sub.5)].sub.n                                   89    5                                         __________________________________________________________________________

                                      TABLE 5                                     __________________________________________________________________________                                 Density,                                                                           Foam                                        Example No.                                                                           Polyphosphazene Blend                                                                              lb/ft.sup.3                                                                        Characteristics                             __________________________________________________________________________    6       [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].s            ub.n /                                                                        [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         45   rigid                                       7       [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].s            ub.n /                                                                        [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-tert                               C.sub.4 H.sub.9)].sub.n                                                                            9    flexible                                    8       [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].s            ub.n /                                                                        [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -tert                    C.sub.4 H.sub.9)].sub.n                                                                            5.2  rigid                                       9       [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-C.sub.9 H.sub.19)].sub.            n /                                                                           [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         31   flexible                                    10      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-sec                   C.sub.4 H.sub.9)].sub.n                                                       [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         24   rigid                                       11      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-sec                   C.sub.4 H.sub.9)].sub.n /                                                     [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         35   flexible                                    12      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-sec                   C.sub.4 H.sub.9)].sub.n /                                                     [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-tert                  C.sub.4 H.sub.9)].sub.n                                                                            11   flexible                                    13      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-C.sub.9               H.sub.19)].sub.n /                                                            [NP(OC.sub.6 H.sub.4 -4-iso C.sub.3 H.sub.7).sub.2 ].sub.n                                         27.1 flexible                                    14      [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-sec C.sub.4 H.sub.9)].s            ub.n /                                                                        [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-OCH.sub.3)].sub.n                                       --   flexible                                    15      [NP(OC.sub.6 H.sub.4 -4-OCH.sub.3) (OC.sub.6 H.sub.4 -4-C.sub.9               H.sub.19)].sub.n /                                                            [NP(OC.sub.6 H.sub.5) (OC.sub.6 H.sub.4 -4-C.sub.2 H.sub.5)].sub.n                                 --   flexible                                    __________________________________________________________________________

EXAMPLE 16

Using the method of Example 5, 4g of [NP(OC₆ H₄ -4-OCH₃) (OC₆ H₄ -4-secC₄ H₉)]_(n) and 6g of [NP(OC.sub. 6 H₄ -4-iso C₃ H₇)₂ ]_(n) wereblended, mixed with the non-polymer ingredients, and foamed. Theresulting foams were flexible and tan in color.

EXAMPLE 17

Using the method of Example 5, 3g of [NP(OC₆ H₄ -4-OCH₃) (OC₆ H₄ -sec C₄H₉)] _(n) and 7g of [NP(OC₆ H₄ -4-iso C₃ H₇)₂ ]_(n) were blended, mixedwith the non-polymer ingredients, and foamed. The resulting foams wereflexible and tan in color.

EXAMPLE 18

Using the method of Example 5, 6g of [NP(OC₆ H₄ -4-OCH₃) (OC₆ H₄ -4-secC₄ H₉) and 4g of [NP(OC₆ H₄ -4-iso C₃ H₇)₂ ]_(n) were blended, mixedwith the non-polymer ingredients, and foamed. The resulting foams wereflexible and tan in color.

EXAMPLE 19

Using the method of Example 5, 7g of [NP(OC₆ H₄ -4-OCH₃) (OC₆ H₄ -4-secC₄ H₉)]_(n) and 3g of [NP(OC₆ H₄ -4-iso C₃ H₇)₂ ]_(n) were blended,mixed with the non-polymer ingredients, and foamed. The resulting foamswere flexible and tan in color.

EXAMPLE 20

Using the method of Example 5, 8g of [NP(OC₆ H₄ -4-OCH₃) (OC₆ H₄ -4-secC₄ H₉)]_(n) and 2g of [NP(OC₆ H₄ -4-iso C₂ H₇)₂ ]_(n) were blended,mixed with the non-polymer ingredients, and foamed. The resulting foamswere flexible and tan in color.

I claim:
 1. The process of foaming a composition comprising a blend ofat least one relatively elastomeric polyphosphazene copolymer having aYoung's Storage Modulus in the range of 1×10⁶ to 5×10⁸ dynes/cm ² andhaving randomly repeating units represented by the formulas##STR6##wherein R₁ and R₂ are the same or different and are hydrogen, aC₁ -C₁₀ linear or branched alkyl radical, or a C₁ -C₄ linear or branchedalkoxy, with the proviso that when R₂ is alkoxy, R₁ and R₂ aredifferent; with at least one relatively stiff or rigid polyphosphazenehomopolymer or copolymer having a Young's Storage Modulus in the rangeof 5×10⁸ to 6×10¹⁰ dynes/cm² and having randomly repeating unitsrepresented by the formulas ##STR7##wherein R₁ and R₂ are the same ordifferent and are hydrogen, a C₁ -C₁₀ linear or branched alkyl radical,or a C₁ -C₄ linear or branched alkoxy, said relatively elastomericcopolymer: relatively stiff or rigid homopolymer or copolymer in a ratioof from about 1:3 to about 3:1, wherein said blend is mixed with achemical blowing agent and the mixture heated to a temperaturesufficient to decompose said blowing agent.
 2. The process of claim 1,wherein at least one of said relatively elastomeric copolymer and saidrelatively stiff or rigid homopolymer or copolymer comprises additionalrandomly distributed repeating units represented by the formulas##STR8##wherein W represents a monovalent radical containing a groupcapable of a crosslinking chemical reaction at moderate temperatures,said group being attached to a P atom by a -O- linkage; the ratio of(OC₆ H₄ -R₁):(OC₆ H₄ -R₂) being from about 1:6 to about 6:1; and theratio of W:[OC₆ H₄ -R₁) + (OC₆ H₄ -R₂)] being less than about 1:5. 3.The process of claim 1 wherein said blowing agent is1,1'-bisazoformamide.
 4. The process of claim 1 which comprises mixingthe blend with a chemical blowing agent and a curing agent and heatingthe mixture to a temperature sufficient to decompose said blowing agentto foam the blend and crosslink the polymers thereof.
 5. The process ofclaim 1 which comprises mixing the blend with a chemical blowing agentand mixture of curing agents, at least one of said curing agents havingan initiation temperature below the temperature at which said chemicalblowing agent decomposes; heating the foamable mass to a temperaturebelow the decomposition temperature of said blowing agent, but above theinitiation temperature of at least one of said curing agents, for asufficient time to partially pre-cure the polymers comprising saidblend, and heating the partially pre-cured mass to a temperature abovethe decomposition temperature of said blowing agent to foam the blendand to effect a further cure thereof.
 6. The process of claim 1 whereinthe Young's Storage Modulus of said relatively elastomericpolyphosphazene copolymer is from about 1×10⁶ to about 5×10⁸ dynes/cm²,and the Young's Storage Modulus of said relatively stiff or rigidpolyphosphazene homopolymer or copolymer is from about 5×10⁸ to about6×10¹⁰ dynes/cm².
 7. The process of foaming the blend of claim 6 whichcomprises mixing the blend with a chemical blowing agent and heating themixture to a temperature sufficient to decompose said blowing agent. 8.The process of foaming the blend of claim 6 which comprises mixing theblend with a chemical blowing agent and a curing agent and heating themixture to a temperature sufficient to decompose said blowing agent tofoam the blend and crosslink the polymers thereof.
 9. The process offoaming the blend of claim 6 which comprises mixing the blend with achemical blowing agent and mixture of curing agents, at least one ofsaid curing agents having an initiation temperature below thetemperature at which said chemical blowing agent decomposes; heating thefoamable mass to a temperature below the decomposition temperature ofsaid blowing agents, but above the initiation temperature of at leastone of said curing agents, for a sufficient time to partially pre-curethe polymers comprising said blend, and heating the partially pre-curedmass to a temperature above the decomposition temperature of saidblowing agent to foam the blend and to effect a further cure thereof.10. The process of claim 1 wherein the Young's Storage Modulus of saidrelatively elastomeric polyphosphazene copolymer is from about 4×10⁶ toabout 7×10⁷ dynes/cm², and the Young's Storage Modulus of saidrelatively stiff or rigid polyphosphazene homopolymer or copolymer isfrom about 2.45×10⁹ to about 2.61×10¹⁰ dynes/cm².
 11. The process offoaming the blend of claim 10 which comprises mixing the blend with achemical blowing agent and heating the mixture to a temperaturesufficient to decompose said blowing agent.
 12. The process of foamingthe blend of claim 10 which comprises mixing the blend with a chemicalblowing agent and a curing agent and heating the mixture to atemperature sufficient to decompose said blowing agent to foam the blendand crosslink the polymers thereof.
 13. The process of foaming the blendof claim 10 which comprises mixing the blend with a chemical blowingagent and mixture of curing agents, at least one of said curing agentshaving an initiation temperature below the temperature at which saidchemical blowing agent decomposes; heating the foamable mass to atemperature below the decomposition temperature of said blowing agent,but above the initiation temperature of at least one of said curingagents, for a sufficient time to partially pre-cure the polymerscomprising said blend, and heating the partially pre-cured mass to atemperature above the decomposition temperature of said blowing agent tofoam the blend and to effect a further cure thereof.
 14. A process offoaming a composition comprising a blend of a relatively elastomericfirst polymer having the general formula

    [NP(OC.sub.6 H.sub.4 -R.sub.1).sub.a (OC.sub.6 H.sub.4 -R.sub.2).sub.b (W).sub.c ].sub.n

wherein R₁ and R₂ are the same or different and are individuallyhydrogen, C₁ -C₁₀ linear or branched alkyl or C₁ -C₄ linear or branchedalkoxy, with the proviso that when R₂ is alkoxy, R₁ and R₂ aredifferent; W represents a monovalent radical containing a group capableof a crosslinking chemical reaction at moderate temperatures, said groupbeing attached to a P atom by a -O- linkage; n is from 20 to 2000; c≧o;a+b+c=2; the ratio of a:b is from about 1:6 to 6:1; the ratio of c:(a+b)is less than about 1:5; and the Young's Storage Modulus of said firstpolymer is in the range of 1×10⁶ to 5×10⁸ dynes/cm² ; with a relativelystiff or rigid second polymer having the general formula

    [NP(OC.sub.6 H.sub.4 -R.sub.1).sub.a (OC.sub.6 H.sub.4 -R.sub.2).sub.b (W).sub.c ].sub.n

wherein R₁ and R₂ are the same or different and are individuallyhydrogen, C₁ -C₁₀ linear or branched alkyl or C₁ -C₄ linear or branchedalkoxy; W represents a monovalent radical containing a group capable ofa crosslinking chemical reaction at moderate temperatures, said groupbeing attached to a P atom by a -O- linkage; n is from 20 to 2000; c≧o;a+b+c=2; the ratio of a:b is from about 1:6 to 6:1; the ratio of c:(a+b)is less than about 1:5; and the Young's Storage Modulus of said secondpolymer is in the range of 5×10⁸ to 6×10¹⁰ dynes/cm² ; the ratio of saidfirst polymer to said second polymer ranging from about 1:3 to about3:1, wherein said blend is mixed with a chemical blowing agent and themixture heated to a temperature sufficient to decompose said blowingagent.
 15. The blend of claim 14 wherein the Young's Storage Modulus ofsaid relatively elastomeric first polymer is from about 1×10⁶ to about5×10⁸ dynes/cm², and the Young's Storage Modulus of said relativelystiff or rigid second polymer is from about 5×10⁸ to about 6×10¹⁰dynes/cm².
 16. The blend of claim 14 wherein the Young's Storage Modulusof said relatively elastomeric first polymer is from about 4×10⁶ toabout 7×10⁷ dynes/cm², and the Young's Storage Modulus of saidrelatively stiff or rigid second polymer is from about 2.45×10⁹ to about2.61×10¹⁰ dynes/cm².
 17. A foamed polymer blend comprised of at leastone relatively elastomeric polyphosphazene copolymer having a Young'sStorage Modulus in the range of 1×10⁶ to 5×10⁸ dynes/cm² and havingrandomly repeating units represented by the formulas ##STR9##wherein R₁and R₂ are the same or different and are hydrogen, a C₁ -C₁₀ linear orbranched alkyl radical, or a C₁ -C₄ linear or branched alkoxy, with theproviso that when R₂ is alkoxy, R₁ and R₂ are different; and at leastone relatively stiff or rigid phosphazene homopolymer or copolymerhaving a Young's Modulus in the range 5×10⁸ to 6×10¹⁰ dynes/cm² andhaving randomly repeating units represented by the formulas##STR10##wherein R₁ and R₂ are the same or different and are hydrogen, aC₁ -C₁₀ linear or branched alkyl radical, or a C₁ -C₄ linear or branchedalkoxy; said first and second polymers being present in said foamedblend in a ratio of from about 1:3 to about 3:1.
 18. The foamed polymerblend of claim 17 wherein the Young's Storage Modulus of said relativelyelastomeric copolymer is from about 1×10⁶ to about 5×10⁸ dynes/cm²,andthe Young's Storage Modulus of said relatively stiff or rigidhomopolymer or copolymer is from about 5×10⁸ to about 6×10⁸ dynes/cm².19. The foamed polymer blend of claim 17 wherein the Young's StorageModulus of said relatively elastomeric copolymer is from about 4×10⁶ toabout 7×10⁷ dynes/cm², and the Young's Storage Modulus of saidrelatively stiff or rigid homopolymer or copolymer is from about2.45×10⁹ to about 2.61×10¹⁰ dynes/cm².
 20. A foamed polymer blendcomprised of a relatively elastomeric first polymer having the generalformula

    [NP(OC.sub.6 H.sub.4 -R.sub.1).sub.a (OC.sub.6 H.sub.4 -R.sub.2).sub.b (W).sub.c ].sub.n

wherein R₁ and R₂ are the same or different and are individuallyhydrogen, C₁ -C₁₀ linear or branched alkyl or C₁ -C₄ linear or branchedalkoxy, with the proviso that when R₂ is alkoxy, R₁ and R₂ aredifferent; W represents a monovalent radical containing a group capableof a crosslinking chemical reaction at moderate temperatures, said groupbeing attached to a P atom by a -O- linkage; n is from 20 to 2000; c≧o;a+b+c=2; the ratio of a:b is from about 1:6 to 6:1; the ratio of c:(a+b)is less than about 1:5; and the Young's Storage Modulus of said firstpolymer is in the range of 1×10⁶ to 5×10⁸ dynes/cm² ; and a secondrelatively stiff or rigid polymer having the general formula

    [NP(OC.sub.6 H.sub.4 -R.sub.1).sub.a (OC.sub.6 H.sub.4 -R.sub.2).sub.b (W).sub.c ].sub.n

wherein R₁ and R₂ are the same or different and are individuallyhydrogen, C₁ -C₁₀ linear or branched alkyl or C₁ -C₄ linear or branchedalkoxy; W represents a monovalent radical containing a group capable ofa crosslinking chemical reaction at moderate temperatures, said groupbeing attached to a P atom by a -O- linkage; n is from 20 to 2000; c≧o,a+b+c=2; the ratio of a:b is from about 1:6 to 6:1; the ratio of c:(a+b)is less than about 1:5; and the Young's Storage Modulus of said secondpolymer is in the range of 5×10⁸ to 6×10¹⁰ dynes/cm² ; the ratio of saidfirst polymer to said second polymer ranging from about 1:3 to about3:1.
 21. The foamed blend of claim 20 wherein c=o.
 22. The blend ofclaim 21 wherein the Young's Storage Modulus of said relativelyelastomeric first polymer is from about 1×10⁶ to about 5×10⁸ dynes/cm²,and the Young's Storage Modulus of said relatively stiff or rigid secondpolymer is from about 5×10⁸ to about 6×10¹⁰ dynes/cm².
 23. The blend ofclaim 21 wherein the Young's Storage Modulus of said relativelyelastomeric first polymer is from about 4×10⁶ to about 7×10⁷ dynes/cm²,and the Young's Storage Modulus of said relatively stiff or rigid secondpolymer is from about 2.45×10⁹ to about 2.61×10¹⁰ dynes/cm².
 24. Thefoamed polymer blend of claim 21 wherein the Young's Storage Modulus ofsaid relatively elastomeric first polymer is from about 1×10⁶ to about5×10⁸ dynes/cm², and the Young's Storage Modulus of said relativelystiff or rigid second polymer is from about 5×10⁸ to about 6×10¹⁰dynes/cm².
 25. The foamed polymer blend of claim 21 wherein the Young'sStorage Modulus of said relatively elastomeric first polymer is fromabout 4×10⁶ to about 7×10⁷ dynes/cm², and the Young's Storage Modulus ofsaid relatively stiff or rigid second polymer is from about 2.45×10⁹ toabout 2.61×10¹⁰ dynes/cm².
 26. The foamed polymer blend of claim 20wherein the Young's Storage Modulus of said relatively elastomeric firstpolymer is from about 1×10⁶ to about 5×10⁸ dynes/cm², and the Young'sStorage Modulus of said relatively stiff or rigid second polymer is fromabout 5×10⁸ to about 6×10¹⁰ dynes/cm².
 27. The foamed polymer blend ofclaim 20 wherein the Young's Storage Modulus of said relativelyelastomeric first polymer is from about 4×10⁶ to about 7×10⁷ dynes/cm²,and the Young's Storage Modulus of said relatively stiff or rigid secondpolymer is from about 2.45×10⁹ to about 2.61×10¹⁰ dynes/cm².
 28. Theprocess of claim 9, wherein the partial pre-cure is accomplished byheating the foamable mass for about 6 to about 30 minutes at atemperature of from about 200° to about 250° F., and the foaming andfurther curing is accomplished by heating the partially pre-cured massfor about 30 minutes to about 60 minutes at a temperature of from about300° to about 350° F.
 29. The process of claim 13, wherein the partialpre-cure is accomplished by heating the foamable mass for about 6 toabout 30 minutes at a temperature of from about 200° to about 250° F.,and the foaming and further curing is accomplished by heating thepartially pre-cured mass for about 30 to about 60 minutes at atemperature of from about 300° to about 350° F.